Generally, when a heart of human body is imaged by an X-ray CT apparatus, the image is diagnostically undesirable when motion artifacts appear on the tomogram due to its heartbeat. To prevent motion artifacts from being generated, it is necessary to highly speed up scanning over one heartbeat cycle to a speed of one heartbeat cycle or less. However, heart rates depend on individual bodies, and physical and mental conditions at the point of imaging in the same body. With a scanner mechanism of the current third-generation CT system, it is hard to highly raise its scan speed to a speed of all heartbeat cycles or less. Therefore, a method is disclosed in Japanese Unexamined Patent Publication 2001-137232 in which an object is imaged not in synchronism with the heartbeat cycle, and data at the same heartbeat time phase are combined after acquiring projection data to reconstruct an image. In this method, electrocardiographic cycle of the object is measured, projection data are acquired by scanning the heart of the object at an angular rate desynchronized with the measured electrocardiographic cycle, and a tomogram of the object's heart is created from segments of temporally discontinuous projection data. The image thus created corresponds to a portion which is selected in the electrocardiographic cycle, e.g., a relatively stationary portion.
However, in the method of producing a cardiac tomogram disclosed in Japanese Unexamined Patent Publication 2001-137232, the following two points are not mentioned.
One is that in some cases a discontinuous portion is generated in projection data necessary for image reconstruction in a longitudinal direction (slicing direction) of the object because in the X-ray CT apparatus having a multi-slice X-ray detector, helical scans are performed, i.e., imaging is performed while an X-ray source and the X-ray detector are transferred and rotated around the object, and temporally discontinuous data are corrected in a projection angle direction (view direction). When the projection data for image reconstruction includes a discontinuous portion, the projection data of a desired slicing position come short. Accordingly, if an image is reconstructed with those data, artifacts are generated on the image or the image quality is significantly deteriorated.
The other one is that when phases of a heartbeat cycle and of a scan cycle are different, the number of data necessary for reconstruction in a projection angle direction it too small to make a reconstruction image and so the image cannot be reconstructed in some cases.
Moreover, when conditions of heartbeat is generally observed, scanning time is conventionally fine-adjusted so that the scan cycle is synchronized with the heartbeat cycle, and cardiac tomograms disclosed in the above Japanese Unexamined Patent Publication 2001-137232 or three-dimensional images obtained from a plurality of cardiac tomograms are sequentially displayed in order of time phase of heartbeat as a moving image. However, in consideration of mechanical accuracy of the scanner which rotates the X-ray source and the X-ray detector around the object, there is a limit on the range of fine adjustment of scanning time. Further, according to the above conventional techniques, one heartbeat is divided and a moving image is produced from images obtained at each heartbeat time phase, thereby it is hard to make a smooth moving image.
Therefore, an object of the present invention is to provide a method of producing a cardiac tomogram and tomograph which can eliminate the discontinuous portion of projection data in the slicing direction and obtain a cardiac tomogram at an arbitrary slicing position with reduced motion artifact caused by heartbeat even when helical scans are performed with an X-ray CT apparatus having a multi-slice detector.
Further, another object of the invention is to provide a method of producing a cardiac tomogram and tomograph which can obtain a cardiac tomogram with few motion artifacts caused by heartbeat and a smoother moving image of cardiac section by solving lack of data number in the projection angle direction necessary for reconstruction even when difference is generated between phases of heartbeat cycle and of scan cycle.